1. Field of the Invention
This invention relates to flexible laminate structures, and in particular to flexible, fluid resistant laminate structures that can withstand a wide range of temperatures and pH values.
2. Description of Related Art
Flexible, fluid resistant materials that can withstand a wide range of temperatures and pHs are required for food processing belts and for expansion joints in factories and power plants. Ever increasingly harsher industrial environments require more robust materials than have heretofore been available.
Methods for continuously processing food products are known and typically have conveyors for transporting food products through a heating process. In some continuous heating conveyor ovens, radiant heat is provided by heated platens over which the belts travel. Such belt materials typically must resist continuous use temperatures of 500° F.-550° F., with infrequent excursions to 600° F.
Productivity is lost if the conveyer system uses belts that degrade quickly. Replacement of conveyer belts and calibration of conveyer systems are time consuming and costly. Downtime is extremely expensive in terms of lost manufacturing yield and may necessitate additional backup lines.
Conveyor belts used for processing food must meet certain FDA standards, including CFR 177.1550, which prohibit the contact of the prepared food with certain materials such as fiberglass. Unfortunately, the conventional conveyor belt used for processing such food is a composite of a brittle woven fiberglass substrate that is dip-coated and saturated in a protective fluoropolymer of PTFE. Scanning electron microscopy (SEM) images of such composite structures provide visual evidence of poor adhesion between the glass rod-like filaments and the PTFE polymer saturating the interior of the composite (FIGS. 1 ands 2). Cross-sectional analysis under SEM magnification show that the glass fibers readily separate from the PTFE and easily fracture.
The oft-called PTFE-coated fiberglass belts are routinely scrapped prematurely due to surface rupture or weak tear resistance, resulting in the exposure of fiberglass to food. Conveyor belts made from PTFE-coated fiberglass last only about a week or two in normal production. The root cause of failure in all instances is the presence of fiberglass.
The woven fiberglass substrate is composed of an industry standard, Style 1528 or 7628 fiberglass fabric with a tear strength (tongue) of about 13 lbs per inch, using ASTM D2261-07. However, the belts are customarily tensioned to at least 1 lbs per inch, which means that over a typical 45 inch wide belt, the total web tension would be 45 lbs. If some or all of that total load is concentrated at a point of tear, it greatly exceeds the tear strength of the fiberglass fabric and the substrate routinely fails.
Likewise for expansion joints, the material of choice has similarly been PTFE coated fiberglass. By definition, expansion joints require flexibility and oftentimes vibration resistance, so fiberglass offers a very limited lifetime because the fiberglass cuts itself to pieces internally. This is visually apparent in FIGS. 3 and 4, where (as a consequence of preparation of the sample for SEM analysis) the cross sectional view displays a jumble of brittle glass fibers, some fractured and the rest left unprotected from grating upon each other during flexure.
Certainly fluoropolymers have previously been applied to PTFE substrates (see, e.g., Bragaw U.S. Pat. No. 4,877,683, Denny U.S. Pat. No. 4,025,679, Fagan U.S. Pat. No. 4,324,574, Griffin U.S. Pat. No. 6,517,919, Kelmartin U.S. Pat. No. 6,770,577, Sassa U.S. Pat. No. 4,983,434), but these have created porous laminates involving porous films and adhesives. PTFE films have been laminated to other films (Tippett, U.S. Pat. No. 7,087,136, Bragaw U.S. Pat. No. 4,877,683), and PTFE film has been shown to be laminated to PTFE coated fiberglass (Tippett, U.S. Pat. No. 7,087,136, Matthiessen U.S. Pat. Pub. 2005/0164581) or suggestively laminated to PTFE woven substrates (Matthiessen U.S. Pat. Pub. 2005/0164581), but at temperatures so high above the melt point of PTFE as to make the material totally inflexible.
The concept of laminating PTFE film to a PTFE substrate to create a flexible, non-porous structure has not been previously revealed for good reason. The PTFE fiber-based substrate is generally not considered a good candidate for the presently envisioned laminate because of the seeming impossibility of laminating it to a fluorocarbon film and achieving a good result. Notwithstanding the lamination difficulties, it is also understood that without proper heat stabilization throughout each process, the fluoropolymer substrate would undergo 20-25% shrinkage when exposed to the 500° F.-550° F. operating temperatures of a food processing belt, resulting in uneven shrinkage within the laminate itself and tracking problems as the belt is driven and rotated.
Therefore, for uses including but not limited to food conveyor belts and expansion joints, a need exists for robust materials that can resist a wide range of temperatures and pH values, and that are tear resistant, fluid resistant, and highly flexible.